In semiconductor lithography (or photolithography), the fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes performed on a semiconductor (for example, silicon) substrate (which is also referred to as a wafer). A photolithography exposure apparatus or scanner is a machine that applies a desired pattern onto a target portion of the substrate. The wafer is fixed to a stage so that the wafer generally extends along a plane defined by orthogonal XL and YL directions of the scanner. The wafer is irradiated by a light beam, which has a wavelength in the deep ultraviolet (DUV) range. The light beam travels along an axial direction, which corresponds with the ZL direction of the scanner. The ZL direction of the scanner is orthogonal to the lateral XL-YL plane. The critical dimension (CD) is the smallest feature size of a pattern that can be printed on the wafer by the exposure apparatus. It is important to maintain a uniform or controlled CD to enable better control over the microelectronic features printed on the wafer.